Control of transmission and two-way signaling systems



S. B. WRIGHT July 2, 1946.

CONTROL 0F TRANSMISSION AND TWO-WAY SIGNALING SYSTEMS Filed May 27, 1939 3 Sheets-Sheet 1 dbh. Q IIE v6 H mi;

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S. B. WRIGHT July 2, 1940.

CONTROL 0F TRANSMISSION AND*TWO-WAY SIGNALING SYSTEMS Filed May 27, 1939 s sheets-sheet 2 @tm u u m .n

/N VEN TOR 5. B. WR/GH T A T TORNE Y July 2, 1940 s. la.V WRIGHT v 2,206,146

CONTROL 0F TRANSMISSION AND TWO-WAY SIGNALING SYSTEMS Filed May 27. 1939 s sheets-sheet s /Nl/ENTOR Bg. B. WR/GHT ATTORNEY to provide improved operation of the system over Fig, l shows a block diagram of a' two-way a wide range of transmission,conditions. radio telephone system embodying the invention; um Alrelated object is to provide maximum gain and 10` l in the transmission paths of a two-way `signal Figs. 2 and 3 show schematically in more detail Patented July 2,1940 y 2,206,146

Num'rEo STATES PATENT t oFEicE CONTROL;` oF TRANSMISSION AND Two-WAY sIGNALINc. sYs'rEMs `Sumner B. Wright,` SouthOrange, N. J., assignor to Bell Telephone Laboratories.,Incorporated, New York, N. Y., a corporation of New York y Application May a7, 1939, serial Nt. 276,112 .513 claims. (c1. 1re- 44;4

` .l The invention relatesto signaling systems, and amount of noise in the receiving path of the particularly to circuits for automatically `conterminal in the absence of received signals.

trolling transmission in such systems.` l The Various objects and features of the in- H An object of the invention is tovcombine and ,venticn will bebetter understood from the fol- 35 relatively arrange automatic devices for exercislowing detailed description thereof read in con- 5 ingdifferent types of control on transmission in junction with the accompanying drawings, in

altwo-way signaling system in such manner as which:

, transmission system `consistent with the mainthe apparatus and circuits which may be used tenance of anti-singing conditions and discrim-` at the twoterminals of a radiotelephone system ination against echoes,`and room and line noises, in acordance with a modification of the invention. and other interference such as that. arising` `in Fig. l shows in single line functional schematic radio paths. l form a two-.way radio telephone system embodyf The invention is particularly `directed to `ciring the invention. In this figure, at each of the 3 cuits forcontrollingthe Volume and direction `of two terminals of the system thereis a two-wire transmission `in a two-way telephone transmistransmission path, the two terminals being joined sion system including yan intermediate circuit by two one-way paths, one for each direction of 20 l subject to considerable noise, suchas a radio link, transmission, and the `transmission apparatus o `and in "which separate frequency bandsor chan` associated with these paths is-identified by sim-- l nels are used for transmission inthe two direcplied representations of their functions or by tions. The probability of'singing in` such a sys-` suitably labeled bOXeS. Contacting arrowheads tem depends primarily on the `operation of eirin a transmission path or box indicates that the cuits` used for automatically p adjusting l volume path at that `point or the apparatus represented levels, which may` at times introduce su-iiicient bythe boxlis normally enabled, and Separated gainto cause over-all or round-trip singing, l In arrowheads in a transmission path `indica-tes that prior" art systems, Voice-operated, `singing sup-` the path atthat point is normally disabled. An pression is usually obtained byauxiliary devices arrow directed towards a normal make point 30 designed to operate overthe range of the Settings in a path 014 TOOX ndeates that the path at that gf thvolume controls. Applicant has improved point orthe transmission apparatus represented on this method of control by making the singing by thebox will be disabled by operation of the f suppression vary with the settings 'of the volume apparatus flOm Whieh the elIOW points, and 5 controls, which in turn Vary with thesignal-toan arrow directed from a box towards a break 35 noise ratios in the intermediate circuit. point in a transmission path indicates that the IIn the circuit of the invention, the anti-singpath Will be enabled et that IJOlt` by Operation ing loss is made a function of thenoise, which of` the weve-Operated devee represented by the "isprimarily responsible for requiring volume conbox. VA box containing a simplied representa' 40 trol and the amount of volume controlfor a partion `oi? a resistance `with an arrow crossing ticular condition is automatically determined by thI'OUgh it represents e Variable OSS 0I' `sain dethe amount of noise in the system. Also, `by Vice, the value of which is adapted to be varied unspecial design and relative arrangement of echo der control of the apparatus from which the arrow suppressor, volume control and noise control ap` D0iI1tS, and e bOX CODtaiTllg Overlapping Circles paratus at the terminals of the system, improved crossed by an arrow, represents a Variable fre- 45 operation is obtained fora wide Variation `of 111161103/ SOllrCe adaptedto be Variedby the aptransmission conditions encountered in Service. paratus from` which the arrow points.

`A feature of the invention "is the use of a The two-way radio telephone system of l'ig.` 1

. control tone transmitted from each terminaltof comprises a west terminal station and an east the system along with the signals and under the terminal station connected by two one-way radio 50 control thereof to control the amount of losslin links. Each station comprises a four-wire cir the transmitting` circuit of the distant terminal cuit connecting a two-way telephone line to a in accordancewith the frequency ofthe control radio transmitter and radio receiver.` As the tone,` the frequency of the tone transmitted fromV circuitstand apparatus at the two terminal sta.-

n each` terminal being `made anunction oi the tionsmare! similar,only that at the west station 55 `the dot-dash box d.

has been illustrated in detail. The four-wire circuit at the west station comprises a transmitting circuit TC leading to a radio transmitter RT and a receiving circuit RC leading from a radio receiver RR, the input of the transmitting circuit TC and the output of the receiving circuit RC being coupled `by the usual hybrid coil H and associated `balancing network Nin substantially conjugate relation with each other and in energy transmitting relation with a two-wire.

circuit I which may be connected by cord circuits or other suitable means in switchboard il :to a twoeway telephone line leading :to a subscribers telephone circuit. A

Directional transmission tover 'the four-wire circuit at the west station vis controlled Yby three different types ci devices working together. At the left are the volume controlsl shown within They include a volume control arrangement comprising a vogad (Volume operated gain adjustingdevce) 5 inthe .trans-n mitting circuit TC anda similar reversevogad .t vin the receiving circuit RC, controlled by a 6060-cycle pilot channel I which is associated .with the vogad andreverse vogad in :suchman`- ner thatwhen the gain `of the former .exceeds a certain value the pilot channel. vadjusts the reverse vogad to put a lcssin the 'receiving cir cuit RC equivalent in decibels to the -increase in gain; and the variable loss switching pads ii and il of optimum values respectively connected in the transmitting circuit TC in ,irontr 'of the vogad 5 and in the receiving .circuit RC li-n front of the reverse vogad 6', which are controlled from the sleeve circuits of .the switchboard 2 to set up the conditions for different classes of calls, i. e., terminal calls or calls that Yare switched through to distant cities.

The vogad 5 and reverse vogad y6 maybe, for example, or the type disclosed in 'B/litchell et ai. Patent 2,019,577, issued'novemter 5, 1935,

and the pilct channel 'I may be as disclosed in British Patent No; 331,831, complete accepted 'Oc-Y tober 13, 1982 (see Fig. 2 and pages '8 to l0).

In the center is an echo suppressor shown within the dot-dash box Ill, including a delay circuit I I in the transmitting circuit TC, a trans k:slitting syllable ampliiier-detector I2 connected across that circuit in front of delay circuit v"I I,

and switching devices (relays) controlled from the output of amplifier-detector i2, operating ina l manner to be described to suppress' echoes and l transmitting circuit TC between "the delay `eircuit Il and the radio transmitter RT, a noise reducer I5 connected in the receiving .circuit RC between the radioreceiver RR and switching pad 9, a variable frequency control current -or tone generator i6 adapted to'be connected 'across the transrmtting circuit TC between delay cir cuit Ii andthe powery reducer Ill, under control of the syllabic amplifier-detector l2 yofthe echosuppressor; and 'a tone-operated receiving control circuit I'i connected to the` receivingcircut RC at a point between the radio receiver 'RR and the noise reducer l5. f

' The tone-operated receiving control circuit Y'l includes the tone selecting lter i8, "and 'cony nected in parallel to vits output, the receiving 2,206,146 y f :0.1.4.5r: ,1

echo suppressor switching device (amplifierd detector) I9, the gain reducer, 2B and the tone filter-integrating device ZI. The switching device I9 controls switching apparatus for suppressing echoes and singing on incoming signals, and

for producing an interlock with other apparatus in the noise controls and the transmitting jecho suppressor. The tone filter-integrating device 42| operates to pass Teceived control tone and as a frequency integrator to adjust the Aloss value of the power reducer Ill in accordance with -thereceived"control tone frequency. The gain .vreducerW-'ZD,operates to measure the applied noise,

and .to adjust the loss or gain value of the noise reducer I5, andthe value of the frequency gen- ,era'ted'by ithe tone generator I6 in accordance with the noise amplitudes.

'The variable frequency tone generator I6 at the west station and the corresponding generator at the east station are selected to generate control wavesoritoneora 'frequency (1n or i2) :which is ynormally 'just :above or just below :the voice frequency band,1the'valueiof'which may-be varied `under control vof thefassociated "fgain reducer 2120.

The mannerin'whic'h the volume control, lecho suppressor and noise `controls 'workvtogether'to attain the objects of the invention 'is-brought out inthe followin'g 'complete description o'fth operation `of 'thesystem ofFig. l'.

.For lca'lls'-\vliich are-connected'to "low loss--orv terminal'land circuits, ythe resistance loss -pads 8 -and lSfare arranged in fthe 'condition indicated in"Fig.'1 with the'padf' connected Vin Ythe-transmitting `circuit TCv to provide a transmitting floss andthe pad 9 short-'circuited iso 'that it' provides little or, no receiving loss.- When extensions are to be 'made through the'terminal switchboard to long via circuits, the sleeve "circuits in switchboard i2 are operated 1to short-circuit theres'istance `pad -8 cutting out fthe transmitting loss', and to remove the Vnormal short Acircuit from resistance pad V'Q'to' put 'in a'receivingloss. :This is in Vorder to Acompensate `for'the additional 'loss in the von une at' the transmitting endend-ais to preserve the stability. The 'optimum values of these 'loss pads for a 'given system `may be readily ascertained vby experiment. v Actually the loss` in the receiving'side may'notjgreatly degrade Ythe connection Vas `will 'be broughtk Vout below. Assuming that the sleeve circuits ofswitchboard 2 have been operated `in 'one ,orthe 'other direction depending `on .the `type of ycalls to be v`handled, the operation 4ofjthe remaining part of 'the systemfor ysuch callswill nowfbe described.

'For either connection "the terminal circuits are normally enabled'in'both directions, and the vogad ii inthe transmitting circuit 'TC andv the reverse vogad S in the receiving circuit RC are in the loss or gain vcondition depending on ,the value to which .'theyhave been adjusted in response to the lspeech currents oi the last speaker. Allso, the loss in the power reducer M in the transmittingcircuit TC which, 'as 1will be dethevlossv inserted' thereby in the receiving circuit if it is necessary.

in proportion to the noisezamplitude, thereby `reducing the volume which will be delivered to the subscriber associated with the terminal to theproper value for the existing noise conditions. The., second accomplishment of the control device 20 actuated by the applied noise is, through the charge produced on condenser 3, to change the frequency of the control `tone generator i6 so that the generated tone is of a particular frequency determined by the amount of noise. Actually, these two results are not accomplished until control tone is received from the distant terminal, the charge on the condenser 3, proportional to the amplitudelevel of the noise applied to the gain reducer 2li being stored therein until the switching device Iii is operatedby received tone to disconnectthe device from` `the condenser at which time condenser 3 discharges to adjust noise reducer I5 and tone generator IB.

Thecorresponding` gain reducer at the east terminal of the system operates similarly to store indications for adjusting the loss value of `the noise reducer in the receiving circuit and the frequency of the control tone generator at that terminal to particular values determined by the amount of noise in the receiving circuit at that station. `Assume now that the west subscriber starts speaking.A His voice waves` will `be impressed by transmitting syllabic amplifier-detector i2 in` the echo suppressor circuit. The portion of t. wests speechenergy applied to the syllable amplifier-detector IZ- will cause its operation to simultaneously close switching contacts 22 to connect the control current generator I6 to the transmitting circuit TC so that control `tone therefrom is impressed on `that circuit; to open the switching contacts 23 inthe receiving circuit RC to suppress echoes in that circuit; and to open switch contacts 24 to disable the output of the receiving echo suppressor device I9 so as to prevent the speech currents of the subscriber associated with the distant east station from obtaining control of the west terminal station while west continues to speak. The delay circuit II in `the `transmitting circuit TC provides sufficient delay in the transmission of lthe voice currents therethrough, so as tojallow the control tone to be slightly `in advance of the voice signals when applied to the radio link.

` The control tone fromsource I6, of a frequency determined by the amount of noise present in the receiving circuit RC at the station, and the following voice currents, pass through the power reducer` I"4 in `which they are attenuated in accordance with its adjustment, to the radio transmitter RT and are radiated thereby. t

, As the control and switching apparatus in the box so labeled at the east terminal station have not been illustrated in detail, their operation in `responsetc` the tone and voice waves received detected in the radio receiver RR at the east station will be described by identifying the control and switching apparatus thereat with the same reference characters as used for the corresponding apparatus at the west terminal station but followed by a prime mark.

1 The incoming control tone and speech waves are separated at the east station by the control tone select lter I8 in the receiving control circuit I'I and the parallel tone reject filter 25 in the receiving circuit RC. The tcnelwaves selected by lter I8 are impressed in parallel on the tone filter-integrating devicel 2|', the amplifier-detector I' and the gain reducer 20'. The tone filter-integrating device 2|' passes the applied control tone and also operates as an integrator to adjust the loss value of the power reducer I4' in the transmitting circuit TC' at the station in accordance with the control tone frequency.

`from the west terminal station and picked up and The portion of the control tone impressed on the amplifier-detector I9' will cause its operation to simultaneously'open switch contacts 2E' to 29', inclusive, and to close switch contacts 30. The opening of switch contacts 2B' will disable the gain reducer in its output thereby discharging condenser 3 to adjust noise reducer I5 and the frequency of the tone generated by thecontrol tone generator IIiin accordance with the noise in thereceiving circuit at the station,

and toprevent operation `of the gain reducer 2li. by the applied tone from chargingcondenser 3'. The opening of switch contacts Z1 will disable thetransmitting circuit TC at a point in front `false operation of these `controls by echoes. The

closing of switch contacts 30 will close a shortcircuiting connnection around the portion of the receiving circuit RC including the noise reducer I5" and the loss pad `ll', thereby effectively cutting out the losses normally inserted `by these elements in the receiving circuit and allowing the received voice currents to pass with little attenuation from the output of filter 25, over the outgoing portion of the receiving circuit RC to the east subscriber. f t

The combined operation of the volume control, echo suppressor` and noise control apparatus at the two terminals of the systemof Fig. 1, as described, results in only enough power output or volume control at the sending end of the system to deliver the necessary signal-to-noise ratio at the receiving end while, by limiting the noise,

`cutting down the received volume at the receivingvend by an` amount necessary for lmaximum intelligibility. Whenever conditions are such` that the circuits operate to take out loss at the sending end of the system, a corresponding amount of loss is put inat the receiving end so that the net effect of this arrangement on singing can be made negligible. The eect of the gain reducer associated withthe tone-operated re` ceiving circuit at each terminal station when control currents and speech are not coming in,

when noise increases at Vthe receiving end, is to increase the loss of the noise reducer in the receiving circuit to prevent the subscriber from hearing so much noise, and to changethe 'frequency 'oi the Acontrol fcurrentfgenerator `at the the circuit of described, vlargely due f to the compensating adjustment in the 'trahir mitting andreceiving sides of .the circuit. At timesof low noise, power is saved in the 'radio transmitter by lhaving a large loss inthe power reducer'while at .the same .timehigh volumes. are deliveredto' the `receiving V:sulfisoriber vbecause of low loss Y.in the inoisezreducer. The arrangement offthewcircuit; so' that incoming control cur- Irentlswitches'out'the*noise reducer loss and any 'loss' which :may be .present .in switching pad il, insures that, even though the noise reducer'los's increases, the received .speech volumeis not materiallyfafiected but the volume of the noise received'lin 'the intervals between speech sounds is reduced.

Preferred circuit arrangements'forthe power reducer.,l noisevreducer, variable frequency tone vgenei-atorx.and the lgain reducer circuit shown diagra matically in li'ig. '1, are illustrated in Figs. 2 and :3"which show schematically the west and east terminal stations respectively of aA twovvay radio telephone system embodying a modiediormzo the invention. As in il'ig. 1, the west and east terminal stations of the system "of Figs. 2 and 3 each includes 'a transmitting circuit TC and a receivingrcireuitRC, the input ofthe former andthe output @of the :latter respectively being coupled byfthe hybrid :,coilvH* and associated balancing network N to a'two-wire circuit l 4which is adapted to be connected through switchboard 2 'to a two-:way -line Yleadingr'to a telephone subscriber. 'rit the' west terminal station of Fig. 2, vthe transmittingacircuit TC includes connected' fbetween the 4hybrid coil i-i vand the radio transmitterRfaremoi/able loss 'pad :3L-the vogad 32 vand the-power reducer 33, and the receiving circuit RQ includes connected between the radio' receiver RR' and the vhybrid coil H, the bandlpas's filter 3d adapted-to pass the speech band and :to rejectavaves of the frequency of the ltonedgransmitt-ed from they `east terminal sta- 'circuit similar to circuit 'l described in connec- .tion'rwithwrig 11,. so that when the vogad gain increasescabove la `certainvalue the reverse vogad yoperates tofinsertfa loss in the receiving .circuit RC equivalent indecibels to the increase in gain `of the vogad.

4Similarly,.at the east-terminal station shown in Fig. 3, the transmitting `circuit YTC includes a removable 4loss lpad 38, a transmitting vvogad A`toarbove for the similarv devices at the Westterminal station,l f y accenna .The arrangement oft-volume control",A echo suppressor and noise control circuits vemployed Aat the west terminal stationshown in Fig.-2 is essentially the same as the similar circuits employed at ,both vterminals 'in the system of Fig. 1, except that the echo suppressor switching circuits in the case of the West terminal shown in Fig. 2 are arrangedto be desensitized by noise. :However, at the east terminal station shown at Fig. 3, the echo suppressor circuits are eliminated and the volume and noise controls employed thereat are essentially the same as described above for the system of Fig. l.

The echo suppressor circuits at the west 'terminal station shown in Fig. 2 comprise a Atransmitting switching circuit l5 `including the tuned amplifier et, the detector il? and the switching` outgoing portion thereof; and the tone4operated receiving-switching circuit 5S includingthe tone selecting lter 5i, the Yone-way ampliiier 52,

the tuned ampliiier 53, fthe detector iid and the switching relays Vsii-toits oontrolledfrom the output of the detector 5t, having itsinput connected across the receiving icircuit RC `in iront of the lter'd. i

Associated with the `transrnitting Vand receiving switching circuits --o thewecho suppressor at the west station of Fig.2 yis a Nogad (noise operated gain adjusting device) shown within the riendash box Et, 4for regulating the sensitivity oi' the 4transmitting receiving echo suppressor switching circuits-according to `the amplitude of the line noise :applied tto .their inputs so as to prevent false operation of these circuits on the 4line noise while maintaining themA at-inaximimi sensitivity for operation on applied tone or speech currents; The particular nega-d device illustrated is essentially as disclosed` and described in Bjornsonpatent '25031,422; issued May 25, y1937 and need only be brieflydescribed here.

The nogad di? includes `a vario-repeater -i 5in the transmitting echo .suppressor circuit 45 :andpa varioerepeater d2 the receivingl .echo suppressor circuit 5t.

The nogad :til also includes -a gain reducer portion S3 'and a isyllabicdisabler portion 64 havingl theirl inputs connected .in 4parallel through a combining 4circuit fit-5, lmay comprise -a `hybrid coil .connection and va ,oommonfamplien such as rdisclosed `in' Bjornson Patent 1,873,268, issued August 23, 1932, to the transmitting suppressor'circuit la in the.outnit of vario-repeater iii, and tof-the receiving suppressor vcircuit 5U in the output of-,the kvario--repeater V62 in such manner as tobe supplied with anzinput from the output of each variorepeater.v

The Ancgad d@ also includes .a marginaldis abler circuitstt connected through a combining circuito?, .similarto .the .combining circuit t5, to the-transmitting suppressor `circuit -ti `in Athe in- Iput of vvariofrepeater Si and tothe receivingfsup.- presser circuity et in theinput ci the vario-repeats er-t in such mannerfas to receive an input .from the yinput of leach vario-repeater; Y

The gain reduceritd,which disclosed in the Bjornson .Patent -.No.- .2.il8l,4l22 may comprise a stage oi :alternating-current amplification, a half? Wave rectiier'fol-lowed -by-:tvvo stages of r-direct ,Ger-fsm.- atiplsaiienf;secretaste chasse-the y.;

gain of the vario-repeaters 6| and t62 in response to the wave energy applied to the `input of the gain reducer in such manner as yto cause a very vslow decrease in the gain of each vario-repeater `when the input energy suddenly increases, i. e., when tone or speech `energy is Vfirst applied to` the `gain reducer and a very rapid increase in gain when the input energy is reduced, i. e., when speech transmission ceases and only noise received from the associated speech transmissie path is v,present in the input.

The disabler circuit 66 comprises a marginal detector 68 and a relay 69 controlled from its output, operating in response to the peaks of the tone or speech impulses applied to the variorepeater 6| or the vario-repeater 62 to disable the gain-reducer circuit 63 in its output thereby preventing excessive reduction in the gain of the i vario-repeaters in response to the strong parts of applied tone or speech impulses.`

` The syllabic disabler circuit 64, which, as indicated, may comprise a sensitive detector 10 with a transformer and a low-pass filter 1| having a cut-off` at about 22 cyclespe'r second in its output,` controlling the operation of the `series electromagnetic switching relays 12 and 13, is substantially` unresponsive to` applied noise waves but responds to the applied weak as well as strong tone or speech impulses inthe input of variorepeater 6| or in the inputof vario-repeater 62 to disable the output of gain reducer 63. The syl-'- labic disablercircuit 64 in connection with the marginal disabler circuit 66 prevents any change in the gain of the vario-repeaters 6! and 62 during a very large part of the time when speech is on the circuit and thereby materially reduces the amount of speech or tone desensitization.

Connected across the transmitting circuit TC between the disabling point therein in front of the power reducer 32 and thehybrid coil H1 is the input of a circuit,` including an amplierdetector 14 and a relay 15, adapted to operate in response toapplied speech waves to connect the output of thevariable frequency control generator 16 to the transmitting circuit TC at a point inthe output of the power reducer 33.

The power reducer 33 comprises two threeelectrode vacuum tubes connected in push-pull Q across the output of amplifier and having a` control condenser 11 in seriesi'n the common portion of the control grid-cathode circuits of the two tubes. Theoutput of a cir= cuitpcorresponding to the tone filter-integrating device 2l in the system of Fig. 1, is connected across condenser 11. This circuit comprises one normally disabled integrating `branch including half-wave vacuum tube rectifier 80 and a lter including a series inductance 8l and a i series condenser 82 connected in series with the 1 control condenser 11, and a parallel branch for enabling the integrating branch, including a detector 18, the sensitivity of which is adjusted so thatit `will be unresponsive to noise but will respend; to the applied tone waves, and `relay 19, the inputs `of both branches being connected 52 in the receiving switching circuit i i l i The noise reducer 36 in the receiving circuit RC atthe west station comprises two three-electrode vacuum tubes connected in push-pull, the com- 1` mon portion of thecontrol gridecathode circuits ofthe two tubes including in series the control, condenserm83, which condenser is also connected across the output of `gain reducer 63 of the nogad 60 through the normallyclosed switching contacts ofrelays 69, 13 and 12. 3 I i The variable frequency control generator 16 comprises a single `three-electrode vacuum tube 84 having its plate connected through a series inductance 85 and series condenser 86` to the control grid of the tube to form an oscillating circuit determining the frequency ofthe generated tone waves. The control'condenser 33, connected in common to the output of the gain reducer 63 and the control grid-cathode circuits of the noise reducer 36, is also connected in series with resistance 81 across the control grid and cathode of the oscillator tube 84, so that its charge will con-` trol the bias on the control grid of tube 84 and thus the frequency generated by tone generator 16. q

As 'will be described, the gain reducer 63 of the nogad 60 besidesoperating to desensitize the echo suppressor switching circuits 45 and 50 in accordance with the noise amplitudes, also operates to perform the same functions as the gain reducer 20 in the system of Fig. l. i i

At the east 'terminal station of the system shown in Fig. 3, the variable frequency control tone generator 88, similar to the control tone generator 16 at the west terminal shown in Fig.

2, is adapted to be connected to the transmitting circuit TC in the output ofthe power reducer 4d by operation of a circuit including ampliiier-dei tcctor 89 and switching relay 9G, connected across a point in front of the tone rejecting'band-pass` filter ll therein is the input of 4a tone-operated receiving switching circuit 9| including thetone `selecting `filter 92 and three control branches,

similar to the receiving control circuit branches provided for the same purpose at the west terminal station of Fig. 2 just described, connected to the output of that filter, respectively operating in the' absence of signal transmission from the station, to adjust the loss value of the noise reducer 43 in the receiving circuit RC and the value of the frequency generated by tone control generator 88 in accordance with the amplitude of the noise passing from `the receiving circuit RC through the narrow band tone selection filter 92, and to control the gain of the power reducer 40 in the transmitting circuit TC at the `station in` accordance with the frequenci7 of tone waves received from the lother station under control of voice signal transmission initiated thereat in a manner whichwill bepdescribed in connection with the following` complete description of operation of the system ofFigs.` 2 and 3.

When no speech signals are being transmitted over the system in either direction, the receiving switching circuits at the west and east stations shown respectively in Figs. 2 and` 3 are adjusted under control of the noise present in the receiving circuits of manner.

At the west station, shown in` Fig.` 2, the narrow band of noise passing through the `tone selecting filter 5| and amplified by the amplifier 52 will be divided between the receiving echo suppressor branch, the nogad 6l) including the gain reducer 63, and the input of the detector 18 in the tone filter-integrating branch. The portion diverted into the receiving echo suppressor bran-ch will not be effective to cause operation `of the echo suppressor relays because the sensitivity of the vario-repeater 62 in that branch is normally adjusted to be inoperative by noise. The portion of thereceived noise waves impressed on detector the station in the following 18' also will have no effect as the sensitivity of that detector is adjusted so thatY itV is unresponfV previously described, and to charge up the condenser 83; Any increase in the charge on the condenser 83, which will be roughly proportional to an increase in some component of the noise applied to the gain reduce-r 63, will increase the normal grid voltages on the tubes of the noise reducer- 35i, reducing its gain accordingly, and will al'sc applya biasing' voltage through resistance Blite the control grid of the tube 3i of the con trol'A tone generator l5 at the station, effectively reducing its generatedI frequency to a value determined by the amount of noise present in the receivingV circuit atA the station. Similarly, ydecrease in the charge on condenser 83 correspondiingto a deer-casein theamcunt-of noise applied' toV gain reducer @3 will proportionately increasethe gain of noise reducer 35 and the frequency f ofthe tone generated by tone generator lt.'

At the east station shown in Fig. 3, the narrow bandi of noise transmitted through the tone se lecting lter 92' and ampliiied by amplier lili' will cause operation of the gain reducer liti to charge the condenser 98 in accordance with the noise amplitudes, thereby controlling` the gain oi thenoise reducer llandv through resistance liti thezbias on the control grid ci the oscillating tube of toneV generator 3B to adjust the value ofthe r generated tone frequency in accordancerwith the amount of' noise in the receiving circuit at the station.

Assume now. that a west subscriber starts speaking from ay telephone near the west station',

so thatfthe loss pad. switching relays' 105 and H35' adapted to be energized 'from the switchboard 2 when the terminal is connected to long via circuits remain in their normally inoperative condition. His voice waves willf pass through the transmitting switching pad 3! and enter the vogad 32 where they will cause its gain to be adjusted to maintainthedesired output volume of the voice signalv current, and at the same time result in a reverse adjustment of the gain ofthe reverse-vogad 311 in the receiving circuit, at the terminal, ifV the gain adjustment exceeds:` th

stability gain requirements.

From the Voutput ety the vogad 32', w'ests voice' waves as regulated in volume by the vogadgvpass' to the hybrid coil H1 in which theyare'r divided, part being transmitted' over the transmission circuit towards thepower reducer and the other portion being diverted into transmitting echo suppressor switchingV circuit l5 through hybrid coil H1. The portion passing over the= main transmission circuit willl be divided between Athe. input of the power reducer 33 and the input of" inl-j I pressed on the amplifier-detector i4 will cause the ampliendetector '14. The portion its operation to operate` relay 15 connecting the output of the control' tone` generator 1S to the circuit TC inthe outputof the 'power reducer 33.

generator 'Hi by the noise waves, as described above, will be transmitted to the radio transmitterl RT and will be radiated to the distant east station. The otherv portion of westsspeech waves The tone waves with a frequency deter-mined by the previous adjustment of the control tone in passing 'through the'. poweru reducer 33:' willv bee attenuated in accordance withlits-v lastiadustment' by-tone waves from the east station, andthe at-v tenuatedl speech vwaves will vfollow the tone waves tof the radio transmitter RT and will also bef radiated to the east station.

Meanwhile, the portion or wests speech wavesalong with the noise waves present' passinginto the transmitting switching lcircuit l5 will betransmitted through the vario-repeater 6l', the' gain of which is adjusted tovary the sensitivity of' switching` circuitv l5 in laccordance with the amountv ofv noise only in circuits l5 and 5d; under control of the circuits 631,155. and 66 of' thencgad til'respectively` connected' to circuit 'l5 onl either sidefof. the vario-repeater El and'tocircuit 5i? on either side of; vario-repeater 52,"in thev manner previously described. 'The speech waves -in the? output' oi vario-repeater Si will pass through tuned amplifier Il@ and detector. il and will cause op..n eration ofi'the switching relayst and til. The operation of 'relay it will connect la disabling. short. circuit across the receiving circuit RC in iront o the delay circuit. Y35. which, will'v serve to: suppress any echo ofwests speech currents whieh'i may be returned'` over the receiving circuit RC at the' west, station. The operation. of switching' relay4 49; will cause. a short;cirouitingA yconnection to bey connected across the receiving: echo; sup:-v pressor switchingy circuitv 5u in front ci the detector' 51?, thereby preventing subsequenty false' operation of the receiving switching relays.' by4 tone energy subsequentlyreceivedj from the' east,Y terminalstation,

Whenthef control. tonefand` the iollowing speech waves from the west station arrive the east? station' shown in Fig. 3, theyj will belpi liedup by' the radio receiver RR'. and will pass' over; the associated" receiving circuit, Thea bandfpass; iilt'erAv si will' suppress the control" tonee but pass` the received. speech waves, whereas the parallel: cone nectedfilter 9! in the receiving switching circuiti- 201 will select the receivedtone and suppress' the received speech, waves oi other frequencies.

' A portion. of thetonewaves4 in.. theyoutput of:

filter 92 will' be diverted` into the ampliiier-deif tector 33. and', will. cause` its; operation to operatively energize theswitchingirelaysxllto 97 con'.-Y nected'V to its output; Relays 94' and 95 will operatarespectively; t'o connectground. tov the grid biasing battery`^ lill for thev tubes of noise reducer ligand to; breali'` theV connection of the gain con'-I trol condenser 93 to the controlgrid-cathode cir'- cnits of thei amplifier tubes in noise reducer 43,'

thereby efectively removing the normal high r negative grid bias on the tubes of the noise re ducer Q3 so; as: to increase its gain. The re'-v sult of this operation is that vvests` speech cur-- rents transmitted through' the noise reducer 43' after passingfthr'ough vthe `ban'd`pass 'lter lll" and: delay circuit l2 are increased in volume'. These speech: currents are then passed through` the reverse vogad and hybrid coil Hto the telephonez circuitsv of the east subscriber associated lter-integrating circuit at the west `terminal station, the elements of this iilter-rectier circuit are selected so that the higher the value of the tone frequency received through the tone filter 921, the greater the voltage appearing across the condenser 99, the eiect being to increase the grid biasing voltage on the power reducer 4|] and thereby reduce its gain in proportionto the in-` crease in the frequency of the control tone, and

vice versa. v i l The charge on Acondenser 8B under control of the gainreducer Idd, which is similar to the nogad gain reducer 63 at the west station,` described above, will be proportional tothe amplitude of the noise applied thereto through amplif` fier I |12 from iilter 92, and `willnot be substantially changed by the applied tone or speech pass= o ing through that lter because of the inherent 4` `slow operation of the reducer on suchwaves, `as] described above.

The applied tone waves, therefore, will not materially change the adjustment of the frequency provided bycondenser QB, o

`The` action of the system of Figs. l and 2 for Hspeech currents transmitted in the opposite direction" is similar. Easts speech currents at the east station of Fig. 3, after passing through the loss pad AStand the vogad 39 will be divided be-` tween the input of the power reducer it in the transmitting circuit and the input ofthe 'ampliiier-detector i9 connected across that circuit in front of the powerreducer. t pressed on the amplifier-"detector 83 willcause its operation to connect the outputof the variablejfrequency tone generator to the transf mitting circuit in the output of power reducer f t 40 softhat the tonewaves generated by that 'generator` with a frequency determined byitsprevious adjustment by lthe noisecurrents in the receiving circuit', described above,` will be transknitted out over that Vcircuit to the radio transrnitter RT in advance of the specchcurrents and will be radiated thereby. lThe speech currents in transmission through the power reducer All will undergo an attenuation determined by its l previous gain adjustment in accordance withthe waves and suppress 1` beamplified by the amplifier t2 and divided be` l radiated thereby. i

frequency of` the tone wave last received from the west terminal station, and will then be transu mitted to the radio transmitter `RT and also The tone and speech waves" from the east terminal `will be piched up and detected by the radio `receiver RRat the west terminal, shown in` Fig. 2and will betransmitted over the rei- 'gceiving circuit RC. `The `band-pass filter 34Min the `receiving speech path will passwthe speech the received tone Whereas the `parallel iilt'er Viii in the receiving switching circuit 50 willl suppress the speech waves and pass the received tone waves.

Thetone waves in the output of liltermil will tween the "receiving control `branches connected I to `the ,output thereof. A portion amplified by l and causethe operation of the switching relays` thus preventing subsequent vfalse operation 1 amplifier 5| will pass through the varirepeater 62,* the tuned amplifier `5t" and the detector be `55 tots: The operation of relays" 59 `and 55 will respectively disable the transmitting echo supe pressor circuit it at a point in front of the detector 47, and to disable thetransmittiiig circuit TC at a point in front oithepower reducer of thewtransmtting suppressor circuit by echoes of easts speech waves and the transmission oisuch` of 'control tone generator 88 `the applied tone t The portion iinu `trol generator condenser` 1'! will windings of relays m5 and radio transmitter RT. The operation of relay 56 will disable the vogad 33, and the operation `of relays 5l and iiwill respectively break'the connection of gain control condenser 33 to the control grid-cathode circuits of the tubes in noise reducer 36 `and `connect ground to thegrid biasing battery |09 for those tubes, thereby eilecI tively removing the high negative grid bias on i the tubesof noise reducer 36 so as to increase its gain.

A portion of the waves in the input of the` vario-repeater '|52 will be transmitted through the combining circuit el into the input of` the marginal disabler circuit of nogad 6B and another portionin the output of the vario-repea`ter E2 will be transmitted through the combining circuit 65 into the input of gain reducer t3 and the input of the syllabic disabler circuit 64 of the nogad. The marginal disabler circuit @E and the` syllabic disabfler `circuit Sit will then operate, in

the manner previously described, to disable the `output of the gain reducer 53 to prevent it from controlling vario-repeaters 62 and 63 to cause desensitization of the transmitting and receiving echo suppressor switching circuits on the applied tone and and speech from varying the charge on condenser `to vary the gain of the noise reducer it in the receiving speech path and the frequency of thelvariable frequency cone I6 at the west terminal. The gain setting of the noise reducer andthe fre` quency ofthe tone generatorl, therefore, will be maintained as previously adjusted bythe gain reducer 53 under control of the noise in the receiving circuit at the terminal.

Another portion of the toneand speech Waves in the output oflampliner 52 in thereceiving switching `circuit 50 impressed on the detector lit will be'detected thereby and cause the operation ofrelay 13 to close a` connection between the output `of amplifier '52 and control condenser l1 through rectifier lill and the lter comprising coil 8| and `condenser 82 in series. The voltage across condenser 71 will then be varied by the applied control tonetvaves in accordance with its frequency, the higher the `frequency the greater` the voltage across the condenser and vice Versa. Theeiect of the variation in the voltage across t be to vary the bias on the grids of thepushpull vacuum tubes in the power reducer 33 in the transmitting circuit IC to change the gain of the power reducer to a value determined by the control tone frequency.

Meanwhile, the received speech waves in the receiving circuit atthe west terminal station will pass through the band-pass lter 34, delay cir#- cuit 35, power reducer 35, reverse vogad 31, and

`hybrid coil H to the listening west subscriber asechoes over the transmitting circuit TC tothe lol speech waves, and to prevent ting circuit of the west station.` `The addition of battery |09 tothecontrol grid circuits of the tubes of noise reducer 3'6 changes `the bias on the grids of these tubes insuch manner as to effectively increase the lossnserted `bythenoise re;v

l from; the, east terminal. 175*- ducerV inithe'receiving: circuit. The. values of. the:

transmitting switch pad 3l' andthe battery ietareselected: so that the loss thus removed from. thetra'nsmitting circuit and. inserted in the re'- c'eiving circuit is suicient to compensate for the additional loss in the connected long distance telephone line. and also to. preserve the stability. Relay 51' is operated by the receiving switching circuit. 50 under control of incoming tone waves to remove the additional battery ills. from` the. grid circuits of the noise reducer 35i and thus to remove the additional loss in the receiving circuit RC `when speech Waves are being received Similar. circuits at the east terminal station of Fig. 3 operate in similar manner to increase the loss inserted; inthe receiving; circuit of that station and: to cut' out the yloss pad 33 from the. transmitting circuit when that terminal is connected to a long distance' telephone line, and to cut out additional loss in the.

receivingcircuitwhen tone Waves are received from the West terminal.

Various modifications ofthe system described above and illustrated in the dravvings'which a-re Within the spirit and scope of the invention will occur to persons skilled the art. For example, in practice it might be'found desirable to combine thefunction of thevogad and power reducer in the: transmitting circuitat each terminal sta- 30; tionfin a single device. Also, it might be advantageous to use switching devices in place of the rela-ys illustrated, which employ dry rectiiiers,

vacuum tubes or other elements operating more.`

gradually on speech than a relay, and to employ noise-desensitized echo suppressor switching circuits at both terminals of a two-way system instead of at only one terminal as illustrated..

What is claimed is:v v if.. In'. a two-Way signal transmission system' ,mi having at each terminalY one-Way transmitting and: receiving circuitsfor. repeating thesignals transmitted in opposite directions, means at each terminal for making the transmitted signal power a function. ofy the amount of noiseV in the receiving circuit of the other terminal, and for making' the` anti-singing loss in the receiving circuit of.

the terminal when signals are being transmitted therefrom a functionof the amount of noise inz the receiving circuit at the same terminal.

2. in combination' with a two-way. signal transmission system. comprising at least near the terminals thereof oppositely directed one-Way repeating pathsA subject to` interfering noise, for

the signals transmitted in opposite directions.,

means inserting an anti-singing lossin one of said paths when signal atransmission. is taking place over the other path, and means to make the valueV of: saidanti-singing loss a function of the amount` of noise in said one path.

lin combination with a two-Way signal tran..- mission system ha'ving' a transmitting signal repeating path. andy a `receiving signal repeating path; at. each terminatsaid pathsv being subject `'to interfering noise, a volume control devicev in the transmitting patlr at a terminal oi system, and means. to make vthe amount of volume control of the transmitted signals exercised by said volume control device a function of the amount of noisein. thereceivingv repeating path at theother terminal.

4. In combination with a two-wayfslgnal transmission system having a transmitting signal repeating path and a receiving signal receiving path subject to interfering noise, at each terminal, volume control` means in the transmitting tion. of the amount of noise in the receiving path of the. other terminal just prior to said signal transmission,

5. In a two-Way signal transmission system having at. the. terminals thereof oppositely di.,- rected one-Way repeating paths for the signalsV transmitted inopposite directions, meansat each terminal responsive to outgoing signals in the transmitting path for causing. analternatingcontrol Wave to be transmitted to the other terminal along with the signals, meansat eachterminal responsive to the control Wave received from.

the other terminal for inserting` a power reducing Aloss in the transmitting path at the rst terminal in an amount dependent on the frequencyl of thel received control Wave and means for. making the frequency of the control -Wave transmitted from each terminal a function of. the amount of noise in the receiving path oftheterminal in thev absence. of received control Waves.

6. In a two-Way signal transmission system', having signal transmitting and signal receivingy circuits at eachv terminal, means for measuring the amount. of noise in the receivingy circuit at a terminal of said system and means. for automaticallyv regulating the. volume of transmitted signals inthe transmitting circuit at the other ter'- minal, in accordance withV the measured value.-

f the noise inthe` rst terminal just prior to the signal transmission.

7. In combination with a two-Way signa-l trans mission systemL having oppositely-directed one-- Way repeatingpathsk at each terminal for the. signals transmitted in opposite directions, each terminal comprising an anti-singing loss inthe receiving path, a source of alternating. control" Waves, means in the absence of signal transmission in either direction over the systenLfor automatically adjusting the. value of said anti-singing loss and the frequency of the control Waves gen.-

verfated by said source in. accordance with the amountof noise in the receiving path at the terminal, control means responsive to the initiation: of signals for transmission. in the transmittingl path to `disable said receiving, circuit, and to cause control Waves from said source to. be transmitted along with the signals to the other terminal, switching means selectively responsive to control Waves received from the other terminalA to adjust the gain of the transmitting circuit in accordance with their frequency, to remove said anti-singing loss from. the receivingA circuit` and to disable the. transmitting circuit and the means for controlling the transmission of con-` trol Waves from the terminal, and means re,-`

sponsive to the signals. initiated in. said transmitting circuit in the absence of prior received. control or signal Waves in the receiving circuit todisable theV selective switching means at thev terminal.

8. In a two-way signal transmission` system having at the terminals thereof oppositely directed one-Way repeating paths for the signals transmitted in opposite directions, means at each terminal responsive to outgoing` signals for. caus..

lof loss in the ing an alternating control Wave to be transmitted to the otherterminal, control means at each terminal for automatically controlling the amount receiving path and the frequency of the transmitted control Wave in accordance with the amount of noisein thereceiving path `at `the same terminal and other control means at each terminal for automatically regulating the gain of the transmitting path in accordance With the frequency of the control wave received from the other terminal whereby the transmitted sigmentioned means comprises a variable frequency i ceiving circuit includes having a biasing circuit, p control means comprises control means at each terminal includes a varicharge of which determines the nal volume at each terminal is a function of the amount of noise in the receiving path at the other terminal. f

9. The system of claim 8 in whichthe iirstcontrol Wave generator having a frequency determining circuit, and means responsive to the out-going signals for connecting said generator tothe transmitting path at the terminal, said rea variable gain amplifier a condenser connected to the frequency determining circuit of said generator` and to the biasing circuit of said amplier so that the frequency of the generated Wave and the gain of said amplifier vary in accordance `with the charge on said condenser and means for chargingsaid condenser in accordancewith the amount of noise in said receiving circuit.

10. `The system of claim 8 in W -ch said other able gain amplifier in the transmitting path, hav-` ing a biasing circuit including a condenser the amplifier gain,

. and frequency integrating means responsive to the received control Wave to vary the charge on and the mst-mentioned nal, means for `singing loss in the receiving circuit at each terminal a function of the amount of noise in that the selected Wave for charging said condenser in c accordance with its frequency.

i2. In a twc-vvay signal transmission system having at the terminals thereof` oppositely dirccted one-Way repeating paths for the signals transmitted in opposite directions, means at each terminal responsive to outgoing signals for sending out to the other terminal a control Wave of a frequency determined by the amount of noise in the receiving path at the other terminal, and means at each terminal responsive to the received control wave for varying the amount of power in outgoing signals in accordance with the frequency of thereceived control Wave.

13. In a two-way signal transmission system comprising transmitting and receiving circuits and associated echo Suppressors at each termi automatically making the anticircuit, the sensitivity of the echo Suppressors at each terminal afunction `of the noise in both the transmitting p circuit and receiving circuit at the terminal, and the transmitted signal power at each terminal a function of the amount of noise in the receiving circuit at the other terminal.

SUMNER B. WRIGHT. 

